The role of the cardiovascular system is to provide an adequate supply of oxygenated blood to the tissues and organs of the body. This function is carefully regulated by the autonomic nervous system (ANS), which continuously senses systemic blood pressure and other hemodynamic variables that reflect the adequacy of tissue perfusion. The input sensed by the ANS is processed by the brain stem, which compares the inputs to desired set points. If the inputs deviate from the set points, the ANS is activated and attempts correct for the difference by bringing about changes in heart rate and vascular tone. This constant feedback and correction by the ANS is reflected in the fluctuations in heart rate (HR) and blood pressure (BP).
Measurements of HR and BP have long been used to characterize an individual's overall cardiovascular health. The measurements have typically consisted of mean values of the HR and BP made at a few points in time; the fluctuations in HR and BP have usually been disregarded. More recently, however, the value and relevance of studying the fluctuations in HR and BP has been demonstrated [1-7]. (Numbers in brackets refer to the references listed at the end of this specification, the teachings of which are incorporated herein by reference.) It has been shown that fluctuations in HR and BP (as measured by their power spectra) and their joint fluctuation (as measured by transfer functions) can provide insight into the level of ANS activity and the physiologic state of the cardiovascular system. This finding has motivated the development of methodologies for using power spectrum and transfer function analysis as a noninvasive physiologic probe [1-7].
An important application for such a methodology is in the evaluation of an older and sicker patient population. Such a population is very likely to have ectopic beats--beats which originate in parts of the heart other than the sinoatrial node (SAN). Whereas the timing of the beats which originate in the SAN is directly modulated by the ANS, the timing and occurrence of ectopic beats is not considered an indicator of cardiovascular feedback and control. From the perspective of power spectrum and transfer function analysis, ectopic beats are a contaminating signal which overshadows the normal activity of the SAN.
The current state of the art is to deal with ectopic beats in one of two ways. One approach is to wait for a sufficiently long interval which is free of ectopic beats [5, 6]. While this method is effective in a younger and healthier population, it is often impractical, if not impossible, in an older and sicker clinical population which has more frequent ectopic beats. The second approach is to estimate what the HR and BP would have been if they had not been obscured by the ectopic beats, and then to substitute those HR and BP estimates in intervals containing ectopic beats. It is common practice to replace the HR and BP by a simple linear spline or weighted average of the points bordering the ectopic beat intervals [4, 7]. However, such simple estimates make unwarranted assumptions about the HR and BP, especially if the ectopic intervals last more than a few seconds.